I had a couple of old analog Kikusui scopes that had an awful pcb layout in the CRT supply, probably 40-50 KV/inch. They would eventually accumulate dust, arc, and zap some exotic diodes [1]. The fix was to clean carefully and glop with thick conformal coating.
John
[1] This was in the cheapie Z-axis coupling circuit. Classic scopes had two transformer-isolated negative supplies, one for the CRT cathode and one for the grid, with the cold ends driven by the intensity control and the unblanking pulse. The cheapie circuit combines AC coupling with a modulated-level ac carrier that's rectified on the high side to achieve sorta-net DC coupling to the grid.